This invention relates to a method for treating process streams to achieve the removal of cyanide, as well as simultaneous removal of heavy metals such as copper, zinc, arsenic, aluminum, lead, nickel, and the like, and precious metals, such as gold, silver, cobalt, platimum, and the like. More specifically, this invention relates to methods and apparatus for removing cyanide, heavy metals and precious metals without destroying the value inherent in those components, utilizing a membrane filtration system, while producing an effluent stream low in contaminants.
Leaching processes (milling or heap) are commonly used to extract gold and other precious metals from ore using various cyanide solutions. The solutions are passed through the ore to leach the precious metals through solubilization, thereby creating a "pregnant" solution, i.e., a solution rich in precious metals. The precious metals are then separated from the pregnant solution. After separation, a "barren" solution remains, but in fact, it is rich in cyanide and still contains low levels of precious metals. Barren solutions are typically reused in the leach process (based upon cyanide content) or treated to destroy and/or remove the cyanide and heavy metals.
Various techniques have been utilized to recover cyanide and heavy and precious metals. Usually, these methods accomplish only one of the objectives at a time to the exclusion of the other values present, for example, where cyanide is destroyed in the process of recovering the precious metals.
Techniques available for removing cyanide by destruction include addition of and reaction with oxidants such as sulfur dioxide, hydrogen peroxide, sodium hypochlorite and chlorine in various forms, as well as metal salts such as copper sulfate, ferrous and ferric salts, and the like, singly or in combination. See, for example, U.S. Pat. Nos. 5,015,396; 4,622,149; 4,851,129 and 4,312,760.
Known methods utilized to remove heavy metals from aqueous solution include precipitation as metal oxide, hydroxide, and oxyhydroxide, adsorpion with or without ion-exchange resins, and the like. Precious metals can be recovered by any number of well known techniques, including ion exchange, carbon adsorption, electrowinning, and zinc precipitation.
The discharge of industrial effluents containing cyanide alone or along with the heavy metal components, are also avoided by their placement in secure storage facilities such as lined ponds and lagoons. But even these present a constant hazard because of possible leaks and percolation into aquifers, spills during periods of heavy rains or snow melts, such as the spring, when the facilities can exceed their expected capacity with the potential of polluting fresh water supplies, and potential wildlife kills such as birds landing on and drinking cyanide laden water.
Consequently, there is a need for a safe, effective and inexpensive method of removing cyanides and cyanide heavy-metal components from aqueous effluents without destroying the values contained therein.
Reverse osmosis and nano filtration are techniques that has been employed to remove dissolved solids from municipal water supplies. For example U.S. Pat. No. 4,574,049 to G. A. Pittner teaches a chemically enhanced reverse osmosis water purification system. But, reverse osmosis has not been well received in the mining industry because the constituents in the process streams including the cyanide compounds tend to foul the membranes employed or may involve pH levels which are too high for the materials of construction used in the membranes or may leave high levels of cyanide or heavy metals in the discharge stream. One example of the use of reverse osmosis in mining is U.S. Pat. No. 4,462,713 to A. K. Zurcher et al, which is directed to a method for mining and reclaiming land. As part of the process, after the major minable products are removed, sodium carbonate is added to the filtrate to remove excess calcium before the filtrate is subjected to reverse osmosis, ion exchange, or electrodialysis to provide fresh water, but without trying to preserve, e.g., cyanide values.